This invention relates to improvements in thermal batteries, and more particularly to improving the operating efficiency of the battery by the incorporation of an additional, independently activatible heat source.
Thermal batteries are primary reserve batteries utilizing an electrolyte which is a nonconductive solid at ambient temperatures. A thermal battery is characterized by having a very long shelf life, in many instances longer than ten years, which can be activated very quickly with little degradation in performance over time. Once activated, the battery supplies electric power from a few seconds to about an hour or longer. No maintenance is required for the thermal battery during storage prior to use, permitting it to be permanently installed in equipment.
The characteristics of the thermal battery permit its use in a number of applications which extend over wide ambient temperature ranges and severe dynamic environments. It is possible for the thermal battery to reliably operate after storage at temperatures ranging from xe2x88x9254xc2x0 C. to 71xc2x0 C. A thermal battery is activated by first activating a supply of heat, such as an ignitable pyrotechnic heat source, which causes melting of the electrolyte thereby activating the one or more cells inside the battery. Because the thermal battery may be subjected to a wide temperature range during storage, it is difficult to anticipate the ambient temperature at the time of activation and therefore the quantity of heat which will be required to melt the electrolyte and activate the cells in the battery. Heat balancing of the battery has thus tended to involve a compromise. Often, the amount of included pyrotechnic heat source material that would provide good performance of the thermal battery at room temperature will yield an excessively high starting temperature in a hot-stored battery, and may lead to thermal runaway. Where the battery is stored at very low temperatures, the same amount of supplied pyrotechnic heat source material may not provide enough heat on activation to obtain an optimal operating temperature.
It has been known to utilize multiple heat sources for a thermal battery. In one configuration, the cell stack contains the primary heat source and a secondary heat source is wrapped around the thermal insulation surrounding the cell stack. The heat from the secondary source reduces the temperature gradient through the thermal insulation from the cell stack interior to the exterior environment, thereby reducing the cell stack cooling rate and extending the battery""s active life. The secondary heat source is generally a zirconium/barium chromatelceramic fiber heat paper.
In the second configuration, a resistance heating element or wire is wrapped around the thermal insulation surrounding the stack. Power to this heating element may be applied to warm a cold-stored battery before use. Alternatively, the thermal battery may be configured to supply the heating element with power during discharge of the thermal battery, thus diverting a portion of the thermal battery output for heating purposes. As before, heat from the secondary source reduces the temperature gradient, thereby reducing the cell stack cooling rate and extending the battery""s active life.
It is an object of the invention to provide an improved thermal battery which provides an input of heat activation energy more closely correlated to the ambient temperature storage conditions of the battery.
It is a further object of the invention to provide an improved thermal battery having a second independently activatible pyrotechnic heat source in addition to a first pyrotechnic heat source to provide heat energy to the battery.
It is yet a further object of the invention to provide an improved thermal battery having a temperature sensor and heat source activating device which will respond to the data collected by the sensor and activate a first pyrotechnic heat source only, or both the first pyrotechnic heat source and a second pyrotechnic heat source, as required by the ambient storage temperature conditions.
The invention is premised on the realization that accounting for the ambient storage temperature of the battery measured at or adjacent the battery core can allow for improved control of thermal battery activation. This control is effected by utilizing a second, independently activatable pyrotechnic heat source in addition to a first pyrotechnic heat source to provide sufficient energy to melt the electrolyte and thereby activate the individual cells of the battery. At least the second pyrotechnic heat source is controlled via a sensor and activating device. The sensor will evaluate the ambient temperature of the battery at or near the core and optionally also outside the battery casing, and determine if the second heat source must be activated in addition to the first heat source to achieve an optimum operating temperature for the cell stack inside the battery casing. The second pyrotechnic heat source may have a different composition with a higher activation temperature than the first heat source, so that it is not activated by the heat generated from the first pyrotechnic heat source or its activating device. Alternatively, if sufficient insulation is provided between the first and second pyrotechnic sources to prevent activation of both when only one is to be activated, each heat source can be produced from the same ignitable material. This latter combination of pyrotechnic sources is advantageously used where one source is positioned inside the battery casing, and the other around the outside perimeter of the casing. Preferably, both the first and second pyrotechnic heat sources are located inside the battery case in close proximity to the electrolyte to more efficiently transfer heat.
A battery having independently ignitable first and second pyrotechnic heat sources wherein at least the second heat source is controlled by a sensor and activating device will be able to at least partially compensate for the ambient temperature of the battery at the time of activation, decreasing both the risk of thermal runaway at excessively high ambient temperature and inefficient operation at low ambient temperature.
Another advantage of a battery with independently ignitable heat sources is the option to activate the electrolyte with one pyrotechnic heat source and allow the cells to partially discharge before the electrolyte cools, and then at a later time reactivate the electrolyte with the second heat source. This application-specific arrangement could allow for reactivation as necessary.
Various other objects and advantages will appear from the following description of the invention and the drawings, in which